We are analyzing the expression of genes active during early embryo development in the frog Xenopus laevis with cloned cDNAs. In a library of 200 cloned sequences that code for abundant RNAs in two embryo stages, we have identified transcripts of some structural and some mitochondrial genes. A significant proportion of cDNAs in this library contain sequences that are moderately repeated in the genome. To analyze stage-specific RNA species we have started to construct a DNA library of particular RNAs which are present in gastrula embryos but absent in oocytes. To analyze the developmental expression of defined genes, we are constructing cDNA clones for two calcium binding proteins, calmodulin and parvalbumin.